Increasingly in many fields, absolute position-measuring devices are being used, in which the absolute position information is derived from a code track having code elements disposed one after the other in the measuring direction. The code elements are distributed pseudo-randomly, so that in each case a certain number of successive code elements forms a bit pattern. A new bit pattern is formed in response to the shift of the scanning unit in relation to the code track by a single code element, and a sequence of different bit patterns is available over the entire measuring range to be recorded absolutely.
A sequential code of this kind is referred to as chain code or pseudo-random code (PRC). A particularly interference-free variant of a pseudo-random code is obtained if the code elements have what is referred to as a Manchester coding, which means that the code elements have two sub-areas of equal size which exhibit properties complementary to each other. The binary information is determined by the sequence of the sub-areas.
German Published Patent Application No. 102 44 235 describes a position-measuring device of this type whose absolute code track is made up of a pseudo-random arrangement of code elements having a Manchester coding. To determine whether the scanning signals of detector elements have values valid for the evaluation of the position information, it is first of all provided to use scanning signals of an incremental track, which extends parallel to the absolute code track, to select the detector elements necessary for the evaluation of the absolute track. Secondly, to evaluate the reliability of the detector signals, it is proposed to split the detector elements into one group having even-numbered detector elements and one group having uneven-numbered detector elements, and in each case to form differential signals of directly succeeding detector elements of each group, and to compare them to a comparison value. The position value is ultimately formed from the valid scanning signals resulting from the comparison.
One widely prevalent functioning principle is optical scanning. In that case, a measuring graduation, which is applied on a measuring standard, is imaged onto a number of photodetectors by directed light emitted from a light source. The measuring standard is movably disposed in the optical path of the light and modulates the light when the measuring graduation is moved relative to the light source and the photodetectors. The position information is ascertained by evaluating the output signals of the photodetectors. The measuring standard is a circular graduation disk or a linear scale, depending on whether a rotary or linear position-measuring device is involved. The measuring graduation may be made up of one or more tracks with regions having different optical characteristics such as transparent/opaque or reflective/non-reflective.
For the reading of the code elements, absolute position-measuring devices whose code track is in the form of a PRC require a great number of detector elements placed relative to each other at an exactly defined distance, which is a function of the code elements to be read. Preferably, the detector elements are combined as a detector array on a semiconductor chip. When working with such an optical scanning principle, it is especially problematic that the reliability of the reading of the code track is dependent on the precision of the imaging of the part of the code track relevant for the reading, onto the detector array. This is a function of the geometrical configuration of the light source, code track and detector elements. For example, if the alignment of the light deviates from that called for, determined by the code track and the detector array, then not all the detector elements used for ascertaining the instantaneous position receive the same quantity of light. Because of this, undefined states may occur and false position values may even be ascertained.